Traditional braking systems for motor vehicles include conventional hydraulic or pneumatic brakes associated with two or more wheels of the vehicle. Such conventional brakes are actuated by pressurized fluid or compressed air. When actuated, the brakes exert a force on a disk or drum which spins in conjunction with the wheel of the vehicle in order to create frictional forces which resist rotation of the wheel. Traditionally, control signals have been transmitted to each of the brake system's actuators mechanically, or by a hydraulic or pneumatic control circuit. However, it has more recently been proposed to employ a centralized control unit to generate electronic control signals and to use such electronic control signals to control actuation of a vehicle's brakes. This type of electronic control scheme has become even more prevalent in view of modern brake systems which now often include not only conventional hydraulic or pneumatic brake actuator functionality, but also supplemental electronic functions such as antilock protection (ABS) and/or electronic braking force distribution (EBV) between the front and rear axles.
U.S. Pat. No. 6,354,671 discloses a brake system in which electronic signals are used to at least partially control actuation of a vehicle's brakes. However, as recognized in the patent, brake system failure due to failure of the electronic control unit is a significant risk. As such, system redundancy is provided in the form of a back-up pneumatic control circuit. Should the electronic control unit malfunction, the braking system is controlled by the back-up pneumatic control circuit in much the same way as traditional brake systems operate. However, such a system suffers from a number of disadvantages. Providing a back-up pneumatic control circuit greatly complicates the braking system and increases the costs thereof. Moreover, when operating in the back-up mode, the advanced functionality of the electronic control system is lost. As such, providing a pneumatic back-up system defeats many of the advantages of providing an electronic control circuit in the first place.
U.S. Pat. No. 6,209,966 obviates some of the problems associated with providing a back-up pneumatic control circuit by employing two electronic control units, which operate independently of each other, and which provide control signals to a brake cylinder assigned to a wheel and a braking pressure modulator valve which is fluid-connected to the brake cylinder. The braking pressure modulator has a first electric actuating element, which can be activated by a first of the two control units, and a second electric actuating element which acts in the same direction when activated as the first electric actuating element. The second electric actuating element can be activated by the second electronic control unit at the same time as the first electric actuating element is being activated by the first electronic control unit. Thus, system redundancy is provided by providing two separate electronic control units, each of which controls one of two separate electric actuating elements associated with each wheel.
While U.S. Pat. No. 6,209,966 obviates some of the problems associated with providing a back-up pneumatic control circuit, it suffers from disadvantages of its own. The braking system disclosed in the '966 patent would require two separate electronic actuating elements associated with each wheel. This requirement, however, needlessly complicates and increases the cost of the system. This is true because control problems, when they arise, are generally caused by a malfunction in the control unit and/or the control network by which control signals are transmitted to the actuating elements, not by failure of the actuating elements themselves. As such, providing two actuating elements for each wheel would not significantly enhance safety of the braking system. Moreover, because both electronic control networks (i.e., the control networks associated with each electronic control unit) are directly connected to actuating elements at each wheel, it is possible for an external catastrophic event, such as a tire explosion, in the vicinity of one of the wheels to cut the network cabling and/or cause a short-circuit in both control networks, thereby causing the entire brake system to fail.
It has also been suggested to create a redundant electronic control system where two separate control networks are employed. Such a system 100, shown in FIG. 1, employs one or more central control units 102 provided to control two or more brake assemblies 104, 106, 108, 110, 112, 114, each having a brake actuator 116 incorporating an electronic control unit 118. Central control unit or units 102 is or are in electrical communication with the electronic control unit 118 of each of brake assemblies 104, 106, 108, 110, 112, 114 via at least two electronic control networks 120, 122. As seen in FIG. 1, all of electronic control units 118 of all brake assemblies 104, 106, 108, 110, 112, 114 are connected to each electronic control network 120, 122. By providing such an arrangement, should one electronic control network fail, the other electronic control network would theoretically maintain control of all brake assemblies.
However, this arrangement suffers from disadvantages similar to those suffered by U.S. Pat. No. 6,209,966 discussed above. More specifically, because both electronic control networks 120, 122 are directly electrically connected to electronic control units 118 of all brake assemblies 104, 106, 108, 110, 112, 114, it is possible for an external catastrophic event, such as a tire explosion, in the vicinity of one of the brake assemblies to cut the network cabling and/or cause a short-circuit in both control networks 120, 122, thereby causing the entire brake system to fail.
What is desired, therefore, is an electrically controlled braking system which is intended for use with wheeled vehicles, which incorporates enhanced safety features, which employs system redundancy in case of partial system failure, which is relatively uncomplicated and less costly as compared to known systems, and which is not prone to complete system failure in the case of an external catastrophic event.